The present invention relates to a process for producing highly concentrated pigment presscakes from which concentrated pigment preparations can be prepared.
Pigment preparations are used to pigment natural and synthetic materials, such as paints, printing inks, plastics, electrophotographic toners, (powder) coating materials, ink-jet inks, color filters, and seed.
One method of preparing such preparations is the flush process, where an aqueous pigment cake is dispersed by means of phase change of the pigments as a consequence of a relatively high affinity of the pigment surface for organic liquids. Phase separation and removal of the residual water with kneading under reduced pressure gives preparations with a low water content.
A different procedure for preparing concentrated pigment preparations starts from powder pigments, which are incorporated together with dispersing auxiliaries into water or organic liquids using a disperser apparatus. Following their synthesis, in which they are obtained predominantly in aqueous systems, the pigments are dried and ground to a pigment powder.
In order for the preparation to contain finely divided pigment particles which are easily wetted with the dispersing medium, the dry pigment powder, which is normally in the form of agglomerates, must be disrupted with high energy input and an extensive duration of dispersion and incorporated into the desired medium in order to ensure sufficient color strength, particularity, and gloss.
In order to prepare pigment preparations having pigment contents of more than 35% by weight of pigment directly from pigment presscakes, the pigment content in the presscake must be more than 50% by weight, preferably more than 55% by weight. Since the low particle size of organic color pigments means that they are usually difficult to filter, they are usually isolated from the mother liquor, following their synthesis, on filter presses of large surface area. This is usually done using membrane filter presses operating with pressures of up to 10 bar. Pigment presscakes from a membrane filter press usually contain not more than from 20 to 40% by weight of pigment and are unsuitable for preparing concentrated pigment preparations. The addition of certain filtration assistant additives, such as the surfactants described in EP-A-0 180 870, for example, allows the pigment content of the presscake to be increased, since the additives employed raise the degree of dewatering by causing agglomeration of the pigment particles. However, these additives are usually a disruptive factor in the dispersion subsequently required for preparing pigment preparations, or in the application medium in which the preparation is to be used.
It is therefore an object of the present invention to develop a process for producing highly concentrated pigment presscakes which overcomes the above-described disadvantages and, in a simple manner, provides pigment presscakes having a pigment content of more than 50% by weight.
In addition, the pigment presscakes produced in this way ought to be readily dispersible in aqueous or organic media in order to allow pigment preparations to be prepared therefrom in an economically advantageous manner.
It has been found that this object can be achieved by the use of a high-pressure filter press without the addition of filtration assistant additives.